Over the past century the demand for energy has grown exponentially. With the growing demand for energy, many different energy sources have been explored and developed. One of the primary sources for energy has been, and continues to be, the combustion of hydrocarbons. However, the combustion of hydrocarbons is usually incomplete combustion and releases non-combustibles that contribute to smog as well as other pollutants in varying amounts.
As a result of the pollutants created by the combustion of hydrocarbons, the desire for cleaner energy sources has increased in more recent years. With the increased interest in cleaner energy sources, fuel cells have become more popular and more sophisticated. Research and development on fuel cells has continued to the point where many speculate that fuel cells will soon compete with the gas turbine generating large amounts of electricity for cities, the internal combustion engine powering automobiles, and batteries that run a variety of small and large electronics.
Fuel cells conduct an electrochemical energy conversion of hydrogen and oxygen into electricity and heat. Fuel cells are similar to batteries, but they can be “recharged” while providing power.
Fuel cells provide a DC (direct current) voltage that may be used to power motors, lights, or any number of electrical appliances. There are several different types of fuel cells, each using a different chemistry. Fuel cells are usually classified by the type of electrolyte used. The fuel cell types are generally categorized into one of five groups: proton exchange membrane (PEM) fuel cells, alkaline fuel cells (AFC), phosphoric-acid fuel cells (PAFC), solid oxide fuel cells (SOFC), and molten carbonate fuel cells (MCFC).
PEM Fuel Cells
The PEM fuel cells are currently believed to be the most promising fuel cell technology, and use one of the simplest reactions of any fuel cell. Referring to FIG. 1, a PEM fuel cell will typically include four basic elements: an anode (20), a cathode (22), an electrolyte (PEM) (24), and a catalyst (26) arranged on each side of the electrolyte (24).
The anode (20) is the negative post of the fuel cell and conducts electrons that are freed from hydrogen molecules such that the electrons can be used in an external circuit (21). The anode (20) includes channels (28) etched therein to disperse the hydrogen gas as evenly as possible over the surface of the catalyst (26).
The cathode (22) is the positive post of the fuel cell, and has channels (30) etched therein to evenly distribute oxygen (usually air) to the surface of the catalyst (26). The cathode (22) also conducts the electrons back from the external circuit to the catalyst, where they can recombine with the hydrogen ions and oxygen to form water. Water is the only by-product of the PEM fuel cell.
The electrolyte (24) is the proton exchange membrane (PEM) (24). The PEM is a specially treated porous material that conducts only positively charged ions. The PEM (24) prevents the passage of electrons.
The catalyst (26) is typically a platinum powder thinly coated onto carbon paper or cloth. The catalyst (26) is usually rough and porous so as to maximize the surface area of the platinum that can be exposed to the hydrogen or oxygen. The catalyst (26) facilitates the reaction of oxygen and hydrogen.
In a working fuel cell, the PEM (24) is sandwiched between the anode (20) and the cathode (22). The operation of the fuel cell can be described generally as follows. Pressurized hydrogen gas (H2) enters the fuel cell on the anode (20) side. When an H2 molecule comes into contact with the platinum on the catalyst (26), it splits into two H+ ions and two electrons (e−). The electrons are conducted through the anode (20), where they make their way through the external circuit (21) that may be providing power to do useful work (such as turning a motor or lighting a bulb (23)) and return to the cathode side of the fuel cell.
Meanwhile, on the cathode (22) side of the fuel cell, oxygen gas (O2) is being forced through the catalyst (26). In some PEM fuel cell systems the O2 source may be air. As O2 is forced through the catalyst (26), it forms two oxygen atoms, each having a strong negative charge. This negative charge attracts the two H+ ions through the PEM (24), where they combine with an oxygen atom and two of the electrons from the external circuit to form a water molecule (H2O).
The PEM fuel cell reaction just described produces only about 0.7 volts, therefore, to raise the voltage to a more useful level, many separate fuel cells are often combined to form a fuel cell stack.
PEM fuel cells typically operate at fairly low temperatures (about 80° C./176° F.), which allows them to warm up quickly and to be housed in inexpensive containment structures because they do not need any special materials capable of withstanding the high temperatures normally associated with electricity production.
Hydrogen Generation for Fuel Cells
As discussed above, each of the fuel cells described uses oxygen and hydrogen to produce electricity. The oxygen required for a fuel cell is usually supplied by the air. In fact, for the PEM fuel cell, ordinary air at ambient conditions is pumped into the cathode. However, hydrogen is not as readily available as oxygen.
Hydrogen is difficult to generate, store and distribute. One common method for producing hydrogen for fuel cells is the use of a reformer. A reformer uses hydrocarbons or alcohol fuels to produce hydrogen, which is then fed to the fuel cell. Unfortunately, reformers are problematic. If the hydrocarbon fuel is gasoline or some of the other common hydrocarbons, then SOx, NOx and other undesirable products are created. Sulfur, in particular, must be removed or it can damage the electrode catalyst. Reformers usually operate at high temperatures as well, which consumes much of the energy of the feedstock material.
Hydrogen may also be created by low temperature chemical reactions utilizing a fuel source in the presence of a catalyst. However, many problems are associated with low temperature chemical reactions for producing hydrogen. One of the primary problems is the requirement for pumps to move the chemical mixture into a reaction chamber filled with a catalytic agent. The use of a pump consumes at least some of the power that the fuel cell is generating (called parasitic power). If the power consumed by the pump becomes too high, the operation of the fuel cell to produce electricity becomes uneconomical.
Further, the chemical mixture provided to the reaction chamber must be accurately metered to facilitate a chemical reaction that will efficiently generate electric power. Accurate metering equipment adds expense, complexity, and sensitivity to the pumping system and increases the parasitic power consumption. Typical fuel cell systems are also usually orientation-specific, meaning that metering of the chemical mixture can only be done when the fuel cell system is in certain orientations. Orientation-specific fuel cell systems limit their usefulness for portable consumer electronics and other devices that may be used in multiple and changing orientations.
In addition, another challenge to using fuel cells in portable consumer products such as digital cameras and laptop computers is providing a hydrogen fuel source that is safe and energy-dense. While there have been fuel cell systems used to generate electricity (such as systems that use the PEM fuel cell described above), they are typically not small or dense enough to be used in most portable consumer products.